Hello Everyone;
A professor here was recently setting up a 4d-hchcnoesy on our Inova 600
(4 RF channels, triple-axis gradients) and uncovered a puzzling problem:
while running the experiment (after optimizing the pulses, water
suppression, etc.) he occassionally observed either Receiver or ADC
Overflow. For at least some of the corresponding FIDs a "glitch" was
apparent. If he turned off C13 decoupling during acquisition, this did not
occur. For testing purposes, we then used a simpler pulse sequence
(somewhat analogous to Varian's pwxcal.c). We set the hard C13 pulse equal
to the total length of all of the hard C13 pulses in the 4d, using the same
power level (63 dB) for that and for the C13 decoupling (47.5 dB) as we had
in the 4d; the acquisition time was 0.05 sec, and d1=1 second. We then
repeatedly ran this 1d experiment (by setting it up as a pseudo-2d
experiment), and occassionally saw "glitches" or differences in the time
domain data from one experiment to the next. If we either turned off the
decoupler or turned down the power for the hard C13 pulse the experiment
was fine (and all the FIDs looked identical). We obtained similar results
with a different decoupling channel.
We then put in a Nalorac probe, and did not observe the problem(s). All
of this suggested to us that the problem was with the probe (especially
since we had just gotten it back from Varian probe repair--C13 arcing at
high power levels).
We sent the probe and data back to Varian, along with the 1d-pulse
sequences and parameter sets, but they have not been able to replicate our
findings. Since they can't find anything wrong with the probe, they are
going to send it back. We'd be very grateful if anyone has any insight or
suggestions as to where to go from here.
Thanks,
Karl Koshlap
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Karl M. Koshlap, Ph.D.
Manager, Biomolecular NMR Laboratory
Department of Biochemistry & Biophysics
Texas A&M University
College Station, TX 77843-2128
Tel: (979) 458-2272
Fax: (979) 845-9274
email: koshlap_at_tamu.edu
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Received on Thu Jan 17 2002 - 17:25:14 MST